Experimental observation of the dual behavior of $\mathcal{P}\mathcal{T}$-symmetric scattering

We investigate experimentally parity-time ($\mathcal{P}\mathcal{T}$) symmetric scattering using $LRC$ circuits in an inductively coupled $\mathcal{P}\mathcal{T}$- symmetric pair connected to transmission line leads. In the single-lead case, the ${\cal PT}$-symmetric circuit acts as a simple dual device - an amplifier or an absorber depending on the orientation of the lead. When a second lead is attached, the system exhibits unidirectional transparency for some characteristic frequencies. This non-reciprocal behavior is a consequence of generalized (non-unitary) conservation relations satisfied by the scattering matrix.